WL#1027: Replace binlog by bin_log in all visible commands and variables

Here are a few commands, options and variables:
SQL_LOG_BIN
SHOW BINLOG EVENTS
--relay-log
--log-bin
--max-binlog-size
--max-relay-log-size
--relay-log-space-limit
--relay-log-purge (in 4.1)

This is not consistent as sometimes 'log' has a '-' beforen sometimes not.
The task is always putting a '-' or '_' before;
SHOW BIN_LOG EVENTS
--max-bin-log-size
Tests and doc will have to be heavily updated.